1. Field of the Invention
This invention relates to a molecular-drag vacuum pump. More particularly, the invention relates to a compact, portable, molecular-drag vacuum pump.
2. State of the Art
In recent years, smaller and more portable chemical and biological sensors have been developed. These sensors have many potential applications, such as in hand-held chemical analyzers, biological detection systems, and other portable sensory instruments. Such instruments may find advantageous use, for example, by soldiers or others to detect the presence of chemical and biological warfare agents; or by inspectors as a simple and rapid means of on-site testing of environmental contaminants; or by law-enforcement personnel testing unknown substances found at a particular location.
However, to fully realize the benefit of these new smaller and more portable sensor systems, relatively compact, low-power vacuum pumps are desirable. Conventional vacuum pumps capable of achieving the desired pumping characteristics are typically too large, and consume too much power, for compatibility with portable sensor systems. Similarly, conventional pumps that are small enough for such applications generally cannot provide the high vacuum typically required for highly accurate sensing and testing of substances at low concentrations. Such conventional pumps are generally ineffective in the Knudsen range, where the concentration of remaining gas molecules is too small for the pump to operate effectively, and yet is the vacuum level where many sensors' effectiveness is enhanced by its provision. Several other solutions to the problem of getting higher vacuum in a small device have been tried, including using cryogenics, absorption of remaining gas molecules by some means, and diaphragm pumps, but, to applicant's knowledge, these have not provided a satisfactory vacuum from a small-enough device. The molecular-drag pump is promising for application in this area.
The concept of the molecular-drag pump was first introduced early in the 20th century, see, e.g. W. Gaede, Annals of Physics, vol. 41, 337 (1913), and was later applied in a disk-shaped version see, e.g. M. Siegbahn, Archives of Mathematics, Astronomy, and Physics, vol. B30, 2 (1944). The basic principle of operation of the molecular-drag pump is to transfer momentum from a high-speed moving surface, such as a rotating rotor, disk or drum, to molecules of a gas, to thereby compress and direct the gas toward an outlet port. One or more wipers are provided to sweep molecules from the rotor toward the outlet, or toward another portion of the rotor in a multi-stage pump, as set forth below. Drag interaction between the moving surface and the gas causes the average kinetic energy of the gas molecules to increase along a pumping path through the pump in contact with the moving surface in a pumping direction; and imparts a net momentum toward the outlet along the path, making the gas as a whole more prone to evacuate the pump through the outlet. In a very low pressure range, this type of pump action causes a larger number of molecules to evacuate a space than other pump types, resulting in a more complete vacuum.
Some pumps of this type have more than one stage. The pumping path contacts a plurality of rotors sequentially, or contacts the same rotor sequentially at a plurality of places. A housing, and/or a housing in combination with wipers, conventionally re-directs the gas molecules sequentially to different locations, or stages, in a multi-stage pump.
Some Design goals regarding small molecular-drag pumps are to make efficient use of the space available for pumping, and to minimize power losses in bearings, in order to achieve a desired performance. In addition, in conventional molecular-drag pumps, the performance can be greatly effected by the tolerance between a wiper and a spinning rotor. Toward these goals, it would be desirable to have a compact molecular-drag pump that eases the fabrication tolerances of the pump parts, yet provides the desired performance. It would also be desirable to have a compact molecular-drag pump that makes use of efficient compact bearings. It is also desirable to have a compact molecular drag pump which compresses the gas in a series of stages in order to sequentially increase the pressure. Finally, it would be desirable to have a multiple-stage molecular-drag pump which accommodates a leakage between pumping stages by directing leakage gas from a later stage into a prior stage to combine with the incoming stream from the prior stage in a pumping direction along the pumping path back into the later stage.